Introduction

Ninja is yet another build system. It takes as input the
interdependencies of files (typically source code and output
executables) and orchestrates building them, quickly.

Ninja joins a sea of other build systems. Its distinguishing goal is
to be fast. It is born from
my
work on the Chromium browser project, which has over 30,000 source
files and whose other build systems (including one built from custom
non-recursive Makefiles) would take ten seconds to start building
after changing one file. Ninja is under a second.

Philosophical overview

Where other build systems are high-level languages, Ninja aims to be
an assembler.

Build systems get slow when they need to make decisions. When you are
in a edit-compile cycle you want it to be as fast as possible — you
want the build system to do the minimum work necessary to figure out
what needs to be built immediately.

Instead, Ninja is intended to be used with a separate program
generating its input files. The generator program (like the
./configure found in autotools projects) can analyze system
dependencies and make as many decisions as possible up front so that
incremental builds stay fast. Going beyond autotools, even build-time
decisions like "which compiler flags should I use?" or "should I
build a debug or release-mode binary?" belong in the .ninja file
generator.

Design goals

Here are the design goals of Ninja:

very fast (i.e., instant) incremental builds, even for very large
projects.

very little policy about how code is built. Different projects and
higher-level build systems have different opinions about how code
should be built; for example, should built objects live alongside
the sources or should all build output go into a separate directory?
Is there a "package" rule that builds a distributable package of
the project? Sidestep these decisions by trying to allow either to
be implemented, rather than choosing, even if that results in
more verbosity.

get dependencies correct, and in particular situations that are
difficult to get right with Makefiles (e.g. outputs need an implicit
dependency on the command line used to generate them; to build C
source code you need to use gcc’s -M flags for header
dependencies).

when convenience and speed are in conflict, prefer speed.

Some explicit non-goals:

convenient syntax for writing build files by hand. You should
generate your ninja files using another program. This is how we
can sidestep many policy decisions.

built-in rules. Out of the box, Ninja has no rules for
e.g. compiling C code.

build-time customization of the build. Options belong in
the program that generates the ninja files.

build-time decision-making ability such as conditionals or search
paths. Making decisions is slow.

To restate, Ninja is faster than other build systems because it is
painfully simple. You must tell Ninja exactly what to do when you
create your project’s .ninja files.

Comparison to Make

Ninja is closest in spirit and functionality to Make, relying on
simple dependencies between file timestamps.

But fundamentally, make has a lot of features: suffix rules,
functions, built-in rules that e.g. search for RCS files when building
source. Make’s language was designed to be written by humans. Many
projects find make alone adequate for their build problems.

In contrast, Ninja has almost no features; just those necessary to get
builds correct while punting most complexity to generation of the
ninja input files. Ninja by itself is unlikely to be useful for most
projects.

Here are some of the features Ninja adds to Make. (These sorts of
features can often be implemented using more complicated Makefiles,
but they are not part of make itself.)

Ninja has special support for discovering extra dependencies at build
time, making it easy to get header dependencies
correct for C/C++ code.

A build edge may have multiple outputs.

Outputs implicitly depend on the command line that was used to generate
them, which means that changing e.g. compilation flags will cause
the outputs to rebuild.

Output directories are always implicitly created before running the
command that relies on them.

Rules can provide shorter descriptions of the command being run, so
you can print e.g. CC foo.o instead of a long command line while
building.

Builds are always run in parallel, based by default on the number of
CPUs your system has. Underspecified build dependencies will result
in incorrect builds.

Command output is always buffered. This means commands running in
parallel don’t interleave their output, and when a command fails we
can print its failure output next to the full command line that
produced the failure.

Using Ninja for your project

Ninja currently works on Unix-like systems and Windows. It’s seen the
most testing on Linux (and has the best performance there) but it runs
fine on Mac OS X and FreeBSD.

If your project is small, Ninja’s speed impact is likely unnoticeable.
(However, even for small projects it sometimes turns out that Ninja’s
limited syntax forces simpler build rules that result in faster
builds.) Another way to say this is that if you’re happy with the
edit-compile cycle time of your project already then Ninja won’t help.

There are many other build systems that are more user-friendly or
featureful than Ninja itself. For some recommendations: the Ninja
author found the tup build system influential
in Ninja’s design, and thinks redo's
design is quite clever.

Ninja’s benefit comes from using it in conjunction with a smarter
meta-build system.

The meta-build system used to
generate build files for Google Chrome and related projects (v8,
node.js). gyp can generate Ninja files for all platforms supported by
Chrome. See the
Chromium Ninja documentation for more details.

Ninja ought to fit perfectly into other meta-build software
like premake. If you do this work,
please let us know!

Running Ninja

Run ninja. By default, it looks for a file named build.ninja in
the current directory and builds all out-of-date targets. You can
specify which targets (files) to build as command line arguments.

There is also a special syntax target^ for specifying a target
as the first output of some rule containing the source you put in
the command line, if one exists. For example, if you specify target as
foo.c^ then foo.o will get built (assuming you have those targets
in your build files).

ninja -h prints help output. Many of Ninja’s flags intentionally
match those of Make; e.g ninja -C build -j 20 changes into the
build directory and runs 20 build commands in parallel. (Note that
Ninja defaults to running commands in parallel anyway, so typically
you don’t need to pass -j.)

Environment variables

Ninja supports one environment variable to control its behavior:
NINJA_STATUS, the progress status printed before the rule being run.

Several placeholders are available:

%s

The number of started edges.

%t

The total number of edges that must be run to complete the build.

%p

The percentage of started edges.

%r

The number of currently running edges.

%u

The number of remaining edges to start.

%f

The number of finished edges.

%o

Overall rate of finished edges per second

%c

Current rate of finished edges per second (average over builds
specified by -j or its default)

%e

Elapsed time in seconds. (Available since Ninja 1.2.)

%%

A plain % character.

The default progress status is "[%f/%t] " (note the trailing space
to separate from the build rule). Another example of possible progress status
could be "[%u/%r/%f] ".

Extra tools

The -t flag on the Ninja command line runs some tools that we have
found useful during Ninja’s development. The current tools are:

query

dump the inputs and outputs of a given target.

browse

browse the dependency graph in a web browser. Clicking a
file focuses the view on that file, showing inputs and outputs. This
feature requires a Python installation. By default port 8000 is used
and a web browser will be opened. This can be changed as follows:

ninja -t browse --port=8000 --no-browser mytarget

graph

output a file in the syntax used by graphviz, a automatic
graph layout tool. Use it like:

ninja -t graph mytarget | dot -Tpng -ograph.png

In the Ninja source tree, ninja graph.png
generates an image for Ninja itself. If no target is given generate a
graph for all root targets.

targets

output a list of targets either by rule or by depth. If used
like ninja -t targets rule name it prints the list of targets
using the given rule to be built. If no rule is given, it prints the source
files (the leaves of the graph). If used like
ninja -t targets depth digit it
prints the list of targets in a depth-first manner starting by the root
targets (the ones with no outputs). Indentation is used to mark dependencies.
If the depth is zero it prints all targets. If no arguments are provided
ninja -t targets depth 1 is assumed. In this mode targets may be listed
several times. If used like this ninja -t targets all it
prints all the targets available without indentation and it is faster
than the depth mode.

commands

given a list of targets, print a list of commands which, if
executed in order, may be used to rebuild those targets, assuming that all
output files are out of date.

clean

remove built files. By default it removes all built files
except for those created by the generator. Adding the -g flag also
removes built files created by the generator (see the rule reference for the generator attribute). Additional arguments are
targets, which removes the given targets and recursively all files
built for them.

If used like ninja -t clean -r rules it removes all files built using
the given rules.

Files created but not referenced in the graph are not removed. This
tool takes in account the -v and the -n options (note that -n
implies -v).

compdb

given a list of rules, each of which is expected to be a
C family language compiler rule whose first input is the name of the
source file, prints on standard output a compilation database in the
JSON format expected
by the Clang tooling interface.
Available since Ninja 1.2.

deps

show all dependencies stored in the .ninja_deps file. When given a
target, show just the target’s dependencies. Available since Ninja 1.4.

recompact

recompact the .ninja_deps file. Available since Ninja 1.4.

Writing your own Ninja files

The remainder of this manual is only useful if you are constructing
Ninja files yourself: for example, if you’re writing a meta-build
system or supporting a new language.

Conceptual overview

Ninja evaluates a graph of dependencies between files, and runs
whichever commands are necessary to make your build target up to date
as determined by file modification times. If you are familiar with
Make, Ninja is very similar.

A build file (default name: build.ninja) provides a list of rules — short names for longer commands, like how to run the compiler — along with a list of build statements saying how to build files
using the rules — which rule to apply to which inputs to produce
which outputs.

Conceptually, build statements describe the dependency graph of your
project, while rule statements describe how to generate the files
along a given edge of the graph.

Syntax example

Here’s a basic .ninja file that demonstrates most of the syntax.
It will be used as an example for the following sections.

Variables

Despite the non-goal of being convenient to write by hand, to keep
build files readable (debuggable), Ninja supports declaring shorter
reusable names for strings. A declaration like the following

cflags = -g

can be used on the right side of an equals sign, dereferencing it with
a dollar sign, like this:

rule cc
command = gcc $cflags -c $in -o $out

Variables can also be referenced using curly braces like ${in}.

Variables might better be called "bindings", in that a given variable
cannot be changed, only shadowed. There is more on how shadowing works
later in this document.

Rules

Rules declare a short name for a command line. They begin with a line
consisting of the rule keyword and a name for the rule. Then
follows an indented set of variable = value lines.

The basic example above declares a new rule named cc, along with the
command to run. In the context of a rule, the command variable
defines the command to run, $in expands to the list of
input files (foo.c), and $out to the output files (foo.o) for the
command. A full list of special variables is provided in
the reference.

Build statements

Build statements declare a relationship between input and output
files. They begin with the build keyword, and have the format
build outputs: rulenameinputs. Such a declaration says that
all of the output files are derived from the input files. When the
output files are missing or when the inputs change, Ninja will run the
rule to regenerate the outputs.

The basic example above describes how to build foo.o, using the cc
rule.

In the scope of a build block (including in the evaluation of its
associated rule), the variable $in is the list of inputs and the
variable $out is the list of outputs.

A build statement may be followed by an indented set of key = value
pairs, much like a rule. These variables will shadow any variables
when evaluating the variables in the command. For example:

If you need more complicated information passed from the build
statement to the rule (for example, if the rule needs "the file
extension of the first input"), pass that through as an extra
variable, like how cflags is passed above.

If the top-level Ninja file is specified as an output of any build
statement and it is out of date, Ninja will rebuild and reload it
before building the targets requested by the user.

Generating Ninja files from code

misc/ninja_syntax.py in the Ninja distribution is a tiny Python
module to facilitate generating Ninja files. It allows you to make
Python calls like ninja.rule(name='foo', command='bar',
depfile='$out.d') and it will generate the appropriate syntax. Feel
free to just inline it into your project’s build system if it’s
useful.

More details

The phony rule

The special rule name phony can be used to create aliases for other
targets. For example:

build foo: phony some/file/in/a/faraway/subdir/foo

This makes ninja foo build the longer path. Semantically, the
phony rule is equivalent to a plain rule where the command does
nothing, but phony rules are handled specially in that they aren’t
printed when run, logged (see below), nor do they contribute to the
command count printed as part of the build process.

phony can also be used to create dummy targets for files which
may not exist at build time. If a phony build statement is written
without any dependencies, the target will be considered out of date if
it does not exist. Without a phony build statement, Ninja will report
an error if the file does not exist and is required by the build.

Default target statements

By default, if no targets are specified on the command line, Ninja
will build every output that is not named as an input elsewhere.
You can override this behavior using a default target statement.
A default target statement causes Ninja to build only a given subset
of output files if none are specified on the command line.

Default target statements begin with the default keyword, and have
the format default targets. A default target statement must appear
after the build statement that declares the target as an output file.
They are cumulative, so multiple statements may be used to extend
the list of default targets. For example:

default foo bar
default baz

This causes Ninja to build the foo, bar and baz targets by
default.

The Ninja log

For each built file, Ninja keeps a log of the command used to build
it. Using this log Ninja can know when an existing output was built
with a different command line than the build files specify (i.e., the
command line changed) and knows to rebuild the file.

The log file is kept in the build root in a file called .ninja_log.
If you provide a variable named builddir in the outermost scope,
.ninja_log will be kept in that directory instead.

Version compatibility

Available since Ninja 1.2.

Ninja version labels follow the standard major.minor.patch format,
where the major version is increased on backwards-incompatible
syntax/behavioral changes and the minor version is increased on new
behaviors. Your build.ninja may declare a variable named
ninja_required_version that asserts the minimum Ninja version
required to use the generated file. For example,

ninja_required_version = 1.1

declares that the build file relies on some feature that was
introduced in Ninja 1.1 (perhaps the pool syntax), and that
Ninja 1.1 or greater must be used to build. Unlike other Ninja
variables, this version requirement is checked immediately when
the variable is encountered in parsing, so it’s best to put it
at the top of the build file.

Ninja always warns if the major versions of Ninja and the
ninja_required_version don’t match; a major version change hasn’t
come up yet so it’s difficult to predict what behavior might be
required.

C/C++ header dependencies

To get C/C++ header dependencies (or any other build dependency that
works in a similar way) correct Ninja has some extra functionality.

The problem with headers is that the full list of files that a given
source file depends on can only be discovered by the compiler:
different preprocessor defines and include paths cause different files
to be used. Some compilers can emit this information while building,
and Ninja can use that to get its dependencies perfect.

Consider: if the file has never been compiled, it must be built anyway,
generating the header dependencies as a side effect. If any file is
later modified (even in a way that changes which headers it depends
on) the modification will cause a rebuild as well, keeping the
dependencies up to date.

When loading these special dependencies, Ninja implicitly adds extra
build edges such that it is not an error if the listed dependency is
missing. This allows you to delete a header file and rebuild without
the build aborting due to a missing input.

depfile

gcc (and other compilers like clang) support emitting dependency
information in the syntax of a Makefile. (Any command that can write
dependencies in this form can be used, not just gcc.)

To bring this information into Ninja requires cooperation. On the
Ninja side, the depfile attribute on the build must point to a
path where this data is written. (Ninja only supports the limited
subset of the Makefile syntax emitted by compilers.) Then the command
must know to write dependencies into the depfile path.
Use it like in the following example:

The -MMD flag to gcc tells it to output header dependencies, and
the -MF flag tells it where to write them.

deps

(Available since Ninja 1.3.)

It turns out that for large projects (and particularly on Windows,
where the file system is slow) loading these dependency files on
startup is slow.

Ninja 1.3 can instead process dependencies just after they’re generated
and save a compacted form of the same information in a Ninja-internal
database.

Ninja supports this processing in two forms.

deps = gcc specifies that the tool outputs gcc-style dependencies
in the form of Makefiles. Adding this to the above example will
cause Ninja to process the depfile immediately after the
compilation finishes, then delete the .d file (which is only used
as a temporary).

deps = msvc specifies that the tool outputs header dependencies
in the form produced by Visual Studio’s compiler’s
/showIncludes
flag. Briefly, this means the tool outputs specially-formatted lines
to its stdout. Ninja then filters these lines from the displayed
output. No depfile attribute is necessary, but the localized string
in front of the the header file path. For instance
`msvc_deps_prefix = Note: including file: `
for a English Visual Studio (the default). Should be globally defined.

If the include directory directives are using absolute paths, your depfile
may result in a mixture of relative and absolute paths. Paths used by other
build rules need to match exactly. Therefore, it is recommended to use
relative paths in these cases.

Pools

Available since Ninja 1.1.

Pools allow you to allocate one or more rules or edges a finite number
of concurrent jobs which is more tightly restricted than the default
parallelism.

This can be useful, for example, to restrict a particular expensive rule
(like link steps for huge executables), or to restrict particular build
statements which you know perform poorly when run concurrently.

Each pool has a depth variable which is specified in the build file.
The pool is then referred to with the pool variable on either a rule
or a build statement.

No matter what pools you specify, ninja will never run more concurrent jobs
than the default parallelism, or the number of jobs specified on the command
line (with -j).

# No more than 4 links at a time.
pool link_pool
depth = 4
# No more than 1 heavy object at a time.
pool heavy_object_pool
depth = 1
rule link
...
pool = link_pool
rule cc
...
# The link_pool is used here. Only 4 links will run concurrently.
build foo.exe: link input.obj
# A build statement can be exempted from its rule's pool by setting an
# empty pool. This effectively puts the build statement back into the default
# pool, which has infinite depth.
build other.exe: link input.obj
pool =
# A build statement can specify a pool directly.
# Only one of these builds will run at a time.
build heavy_object1.obj: cc heavy_obj1.cc
pool = heavy_object_pool
build heavy_object2.obj: cc heavy_obj2.cc
pool = heavy_object_pool

The console pool

Available since Ninja 1.5.

There exists a pre-defined pool named console with a depth of 1. It has
the special property that any task in the pool has direct access to the
standard input, output and error streams provided to Ninja, which are
normally connected to the user’s console (hence the name) but could be
redirected. This can be useful for interactive tasks or long-running tasks
which produce status updates on the console (such as test suites).

While a task in the console pool is running, Ninja’s regular output (such
as progress status and output from concurrent tasks) is buffered until
it completes.

Ninja file reference

A file is a series of declarations. A declaration can be one of:

A rule declaration, which begins with rule rulename, and
then has a series of indented lines defining variables.

A build edge, which looks like build output1output2:
rulenameinput1input2.
Implicit dependencies may be tacked on the end with |
dependency1dependency2.
Order-only dependencies may be tacked on the end with ||
dependency1dependency2. (See the reference on dependency types.)

Implicit outputs (available since Ninja 1.7) may be added before
the : with | output1output2 and do not appear in $out.
(See the reference on output types.)

Lexical syntax

Ninja is mostly encoding agnostic, as long as the bytes Ninja cares
about (like slashes in paths) are ASCII. This means e.g. UTF-8 or
ISO-8859-1 input files ought to work.

Comments begin with # and extend to the end of the line.

Newlines are significant. Statements like build foo bar are a set
of space-separated tokens that end at the newline. Newlines and
spaces within a token must be escaped.

There is only one escape character, $, and it has the following
behaviors:

$ followed by a newline

escape the newline (continue the current line
across a line break).

$ followed by text

a variable reference.

${varname}

alternate syntax for $varname.

$ followed by space

a space. (This is only necessary in lists of
paths, where a space would otherwise separate filenames. See below.)

$:

a colon. (This is only necessary in build lines, where a colon
would otherwise terminate the list of outputs.)

$$

a literal $.

A build or default statement is first parsed as a space-separated
list of filenames and then each name is expanded. This means that
spaces within a variable will result in spaces in the expanded
filename.

Other whitespace is only significant if it’s at the beginning of a
line. If a line is indented more than the previous one, it’s
considered part of its parent’s scope; if it is indented less than the
previous one, it closes the previous scope.

Top-level variables

Two variables are significant when declared in the outermost file scope.

(Available since Ninja 1.3.) if present, must be one of
gcc or msvc to specify special dependency processing. See
the full discussion. The generated database is
stored as .ninja_deps in the builddir, see the discussion of builddir.

msvc_deps_prefix

(Available since Ninja 1.5.) defines the string
which should be stripped from msvc’s /showIncludes output. Only
needed when deps = msvc and no English Visual Studio version is used.

description

a short description of the command, used to pretty-print
the command as it’s running. The -v flag controls whether to print
the full command or its description; if a command fails, the full command
line will always be printed before the command’s output.

generator

if present, specifies that this rule is used to
re-invoke the generator program. Files built using generator
rules are treated specially in two ways: firstly, they will not be
rebuilt if the command line changes; and secondly, they are not
cleaned by default.

in

the space-separated list of files provided as inputs to the build line
referencing this rule, shell-quoted if it appears in commands. ($in is
provided solely for convenience; if you need some subset or variant of this
list of files, just construct a new variable with that list and use
that instead.)

in_newline

the same as $in except that multiple inputs are
separated by newlines rather than spaces. (For use with
$rspfile_content; this works around a bug in the MSVC linker where
it uses a fixed-size buffer for processing input.)

out

the space-separated list of files provided as outputs to the build line
referencing this rule, shell-quoted if it appears in commands.

restat

if present, causes Ninja to re-stat the command’s outputs
after execution of the command. Each output whose modification time
the command did not change will be treated as though it had never
needed to be built. This may cause the output’s reverse
dependencies to be removed from the list of pending build actions.

rspfile, rspfile_content

if present (both), Ninja will use a
response file for the given command, i.e. write the selected string
(rspfile_content) to the given file (rspfile) before calling the
command and delete the file after successful execution of the
command.

This is particularly useful on Windows OS, where the maximal length of
a command line is limited and response files must be used instead.

Interpretation of the command variable

On Unixes, commands are arrays of arguments. The Ninja command
variable is passed directly to sh -c, which is then responsible for
interpreting that string into an argv array. Therefore the quoting
rules are those of the shell, and you can use all the normal shell
operators, like && to chain multiple commands, or VAR=value cmd to
set environment variables.

On Windows, commands are strings, so Ninja passes the command string
directly to CreateProcess. (In the common case of simply executing
a compiler this means there is less overhead.) Consequently the
quoting rules are deterimined by the called program, which on Windows
are usually provided by the C library. If you need shell
interpretation of the command (such as the use of && to chain
multiple commands), make the command execute the Windows shell by
prefixing the command with cmd /c.

Build outputs

There are two types of build outputs which are subtly different.

Explicit outputs, as listed in a build line. These are
available as the $out variable in the rule.

This is the standard form of output to be used for e.g. the
object file of a compile command.

Implicit outputs, as listed in a build line with the syntax |
out1out2 + before the : of a build line (available since
Ninja 1.7). The semantics are identical to explicit outputs,
the only difference is that implicit outputs don’t show up in the
$out variable.

This is for expressing outputs that don’t show up on the
command line of the command.

Build dependencies

There are three types of build dependencies which are subtly different.

Explicit dependencies, as listed in a build line. These are
available as the $in variable in the rule. Changes in these files
cause the output to be rebuilt; if these file are missing and
Ninja doesn’t know how to build them, the build is aborted.

This is the standard form of dependency to be used e.g. for the
source file of a compile command.

Implicit dependencies, either as picked up from
a depfile attribute on a rule or from the syntax | dep1dep2 on the end of a build line. The semantics are identical to
explicit dependencies, the only difference is that implicit dependencies
don’t show up in the $in variable.

This is for expressing dependencies that don’t show up on the
command line of the command; for example, for a rule that runs a
script, the script itself should be an implicit dependency, as
changes to the script should cause the output to rebuild.

Note that dependencies as loaded through depfiles have slightly different
semantics, as described in the rule reference.

Order-only dependencies, expressed with the syntax || dep1dep2 on the end of a build line. When these are out of date, the
output is not rebuilt until they are built, but changes in order-only
dependencies alone do not cause the output to be rebuilt.

Order-only dependencies can be useful for bootstrapping dependencies
that are only discovered during build time: for example, to generate a
header file before starting a subsequent compilation step. (Once the
header is used in compilation, a generated dependency file will then
express the implicit dependency.)

File paths are compared as is, which means that an absolute path and a
relative path, pointing to the same file, are considered different by Ninja.

Variable expansion

Variables are expanded in paths (in a build or default statement)
and on the right side of a name = value statement.

When a name = value statement is evaluated, its right-hand side is
expanded immediately (according to the below scoping rules), and
from then on $name expands to the static string as the result of the
expansion. It is never the case that you’ll need to "double-escape" a
value to prevent it from getting expanded twice.

All variables are expanded immediately as they’re encountered in parsing,
with one important exception: variables in rule blocks are expanded
when the rule is used, not when it is declared. In the following
example, the demo rule prints "this is a demo of bar".

Evaluation and scoping

Top-level variable declarations are scoped to the file they occur in.

Rule declarations are also scoped to the file they occur in.
(Available since Ninja 1.6)

The subninja keyword, used to include another .ninja file,
introduces a new scope. The included subninja file may use the
variables and rules from the parent file, and shadow their values for the file’s
scope, but it won’t affect values of the variables in the parent.

To include another .ninja file in the current scope, much like a C
#include statement, use include instead of subninja.

Variable declarations indented in a build block are scoped to the
build block. The full lookup order for a variable expanded in a
build block (or the rule is uses) is:

Special built-in variables ($in, $out).

Build-level variables from the build block.

Rule-level variables from the rule block (i.e. $command).
(Note from the above discussion on expansion that these are
expanded "late", and may make use of in-scope bindings like $in.)

File-level variables from the file that the build line was in.

Variables from the file that included that file using the
subninja keyword.